Currently, digital images are derived through various devices including digital cameras and scanners for digitally scanning of film images. Many times the sharpness of an image is degraded by optical elements or by irregularities in the image sensor. For these reasons, it is often desirable to sharpen an image after it has been converted to a digital form. Conventional sharpening methods, such as unsharp masking, achieve the appearance of edge sharpening by locally lightening the lighter portion of an edge region and locally darkening the darker portion of an edge region. The resulting increase in microcontrast provides the sharpening effect. Such methods can be applied to black and white digital images as well as to colored digital images.
Referring to FIG. 1A, a one dimensional trace of an edge profile is shown in which higher code values correspond to lighter shades and lower code values to darker shades. In FIG. 1B, the same edge profile has been further blurred in accordance with the prior art technique of unsharp masking. The curve in FIG. 1B is subtracted from the curve in FIG. 1A and the resulting curve shown in FIG. 1C in which the amplitudes P and N, for positive and negative boost respectively, are approximately the same size. The difference curve of FIG. 1C is added to the original curve in FIG. 1A and this final curve, shown in FIG. 1D, depicts the profile of the sharpened edge. Although unsharp masking was originally a film technique, it also has a digital version. Shown in FIGS. 2A-C are examples of boost kernels which, when applied to a digital image, directly produce boost values analogous to those shown in FIG. 1C and sharpened edges analogous to those shown in FIG. 1D.
When these conventional sharpening methods are applied too aggressively, image artifacts will appear. One artifact is an unnatural (light) halo appearing in the lighter region of an edge. Another artifact is an unnatural dark outline appearing in the darker region of an edge. It is typically that the positive boost required to create the halo artifact is smaller (in absolute value) than the negative boost required to create the outline artifact.
An additional step often taken in digital sharpening is the application of a coring function to reduce the amplification of noise. Three examples of (prior art) coring functions are shown in FIGS. 3A-C. Each initial boost value is used as input to the coring function and the resulting output is the cored boost value. The cored boost value is then added to the original image. The three sample coring functions all show a flat region at the origin which maps any small boost value to zero. These coring functions also exhibit a symmetry which insures that the gains applied to positive and negative boost values are the same. Whether a coring function is used or not, the problem is that the halo artifact will appear for boost levels which are still producing beneficial results in darker edge regions. Consequently, the practical limit of image sharpening is almost always determined by the appearance of the halo artifact.